1. Field of the Invention
The present invention relates to a method for producing a magneto-optical recording medium capable of recording a recording signal by the optical intensity modulation direct overwriting.
2. Description of the Prior Art
In the field of magneto-optical recording and reproduction, a request for a higher transfer rate has been raised. In order to answer this request, various techniques have been developed and are being implemented in practice. Among them, there can be named a technique of light-intensity modulation direct overwriting.
When recording data on an magneto-optical recording medium with a light-intensity modulation method, normally, a weak DC magnetic field is applied to the magneto-optical recording medium and a laser beam is radiated while modulating the beam depending on the presence/absence of a signal. Consequently, when re-recording a data on a already recorded area by using a conventional magneto-optical recording/reproduction apparatus of the light-intensity modulation method, it is necessary to erase the recorded area prior to the re-recording. That is, it is impossible to directly overwrite on the already recorded area.
Thus, most of the magneto-optical recording/reproduction apparatuses which have been implemented in practice require an erase operation prior to recording a new data. This means that the magneto-optical recording medium should rotate by at least two turns for recording a signal, which has prevented to provide a high transfer rate.
In order to solve such a problem, there has been designed the light-intensity modulation direct overwriting technique which employs the light-intensity modulation method and enables direct overwriting.
For example, Japanese Patent Laid-Open Sho 62-175948 discloses a light-intensity modulation direct overwriting method which employs an magneto-optical recording medium made from two magnetic layers layered having different magnetic characteristics and layered so as to be connected to each other by exchange interaction; and an magneto-optical recording and reproduction apparatus including: an optical system capable of radiating a beam controlled to two values, i.e., a high level and a low level, during a recording; a recording magnetic field generating apparatus which is used in an ordinary magneto-optical recording and reproducing apparatus; and a so-called initialization magnet, i.e., an external magnetic field generating apparatus capable of inverting only one of the two layers at a room temperature.
However, in order to realize a light-intensity modulation direct overwriting by using this technique, the initialization magnet should an initialization magnetic field in the order of several kOe, which is a problem in designing the magneto-optical recording and reproduction apparatus.
In order to cope with this, reduction of the initialization magnetic field has been tried such as in a method disclosed in Japanese Patent Laid-Open Hei 1-185853. Such efforts have reduced the initialization magnetic field in the order of 2 kOe, which is still a significantly great value if compared to the recording magnetic field in the order of 300 Oe. This initialization magnetic field is still a problem and this method is not yet implemented in practical use.
On the other hand, separately from the examination for reduction of the initialization magnetic field, a research has been made to enable the light-intensity modulation direct overwriting without using any initialization magnetic field. As a result, there has been designed an magneto-optical recording medium disclosed in Japanese Patent Sho 63-268103 and further in Japanese Patent Hei 3-219449.
This magneto-optical recording medium basically consists of the magneto-optical recording medium disclosed in Japanese Patent Laid-Open Sho 62-175948 with addition of a magnetic layer which serves as an initialization magnet by the exchange interaction force. This magnetic layer is prepared in such a manner that its magnetization will not change during a recording and is called an initialization layer. As can be understood from its role, the initialization layer is once magnetized in a predetermined direction after preparation of the medium, and after that its magnetization direction should not be inverted.
Consequently, in order to satisfy the condition that the initialization layer does not invert its magnetization direction during a direct overwriting, the initialization layer should have magnetic characteristics that the Curie temperature is sufficiently high and the product of the coercive force Hc and the saturation magnetization Ms is sufficiently great. As a material satisfying such magnetic characteristics, there can be exemplified TbFeCo (Co content is 50 atomic % or above) or the like. Examination has been made on a magneto-optical recording medium using such a material as the initialization layer.
The aforementioned initialization layer should be once magnetized in a predetermined direction after preparation of the medium, and it should satisfy the requirement that it can be polarized at a room temperature. For this, the initialization layer should have a coercive force Hc of ten and several kOe at a room temperature.
The TbFeCo which has been examined as the material of the initialization layer satisfies the conditions that the Curie temperature is sufficiently high and the product of the coercive force Hc and the saturation magnetization Ms is sufficiently great. However, there arise various problems when these magnetic characteristics are to be obtained simultaneously with satisfying the requirement that the coercive force Hc is ten and several kOe or below at the room temperature.
An amorphous of a rare earth element and a transition metal element such as TbFeCo shows at the compensation composition the infinite coercive force Hc, and the coercive force Hc is reduced when changing the composition ratio of the rare earth element and the transition metal element. In general, the composition containing an increased ratio of the rare earth element compared to the compensation composition is called RE-rich, whereas the composition containing an increased ratio of the transition metal element is called TM-rich. In other words, the coercive force Hc is reduced when the compensation composition is changed into any of the directions, i.e., into the RE-rich direction or into the TM-rich direction. Consequently, the composition satisfying the requirement that the coercive force Hc is ten and several kOe or below at the room temperature exists both in the RE-rich side and in the TM-rich side.
On the other hand, in the amorphous of rare earth and transition metal containing a plenty of Co such as TbFeCo containing 50 atomic % of Co or above, the Curie temperature greatly depends on the ratio between the rare earth element and the transition metal element, i.e., the Curie temperature is increased as the ratio of transition metal element is increased. If this situation is considered, in order to simultaneously satisfy the conditions that the Curie temperature is sufficiently high and the coercive force Hc is ten and several kOe at the room temperature, it is considered to select a composition of TM-rich having a coercive force Hc of ten and several kOe at the room temperature.
However, as this material shows the Curie temperature which is very high, in the composition of TM-rich satisfying the condition that the coercive force Hc is ten and several kOe at the room temperature, the magnetic anisotropy is lowered at a higher rate compared to the magnetization increase as the temperature increases, i.e., the magnetization direction is put into disorder below the Curie temperature. For this, conventionally, when using the amorphous of rare earth and transient metal such as TbFeCo as the initialization layer, there has been no other selection than to use a composition of RE-rich having a coercive force Hc of ten and several kOe at the room temperature.
Note that the Curie temperature is generally increased when the Co content is increased, and it is also considered to compensate the Curie temperature which has been lowered, with addition of Co. However, addition of Co which exhibits a great effect in the TM-rich composition exhibits only a small effect in the RE-rich composition. For this, addition of Co cannot sufficiently increase the Curie temperature of the initialization layer.
As has thus far been described, in the magnet-optical recording medium capable of light intensity modulation direct overwriting by providing the initialization layer, the initialization layer should simultaneously satisfy the conditions that the Curie temperature is sufficiently high, the product of the coercive force Hc and the saturation magnetization Ms is sufficiently great, and the coercive force Hc at the room temperature is ten and several kOe or below. However, it has been difficult to satisfy these conditions and to obtain an initialization layer having a high stability against the magnetization inversion during the light intensity modulation direct overwriting.
It is therefore an object of the present invention to provide a method for producing a magneto-optical recording medium capable of light intensity modulation direct overwriting, the method enabling to easily form an initialization layer having an excellent stability against the magnetization inversion during the light intensity modulation direct overwriting.
In order to achieve the aforementioned object, the present invention provides a method for producing a magneto-optical recording medium comprising: at least a magnetic layer which is magnetized according to a recording signal; a magnetic layer whose magnetization direction is temporarily changed according to a recording signal during a recording; a magnetic layer which is temporarily demagnetized during a recording; and a magnetic layer whose magnetization direction is not changed during a recording; wherein the magnetic layer which is magnetized according to a recording signal consists of at least a first magnetic layer in which residual magnetization Mr is smaller than saturation magnetization Ms, and a second magnetic layer in which a ratio Mr/Ms between a residual magnetization Mr and a saturation magnetization Ms is greater than that of the first magnetic layer; and an external magnetic field is applied in an almost vertical direction to a film surface at least when preparing the second magnetic layer on the first magnetic layer. Here, the aforementioned external magnetic field is preferably applied when the second magnetic layer has reached 0.5 nm thickness or above.
As has been described above, according to the present invention, an external magnetic field is applied in a vertical direction to a film surface when forming the second magnetic layer on the first magnetic layer. Thus, the magnetic layers are polarized. The magnetic layers which are formed afterwards are successively connected by exchange interaction and accordingly, the magnetization direction in each of the magnetic layers is not in disorder but oriented in a particular direction. Consequently, according to the present invention, the magnetic layer whose magnetization direction is not changed during a recording, i.e., the initialization layer is polarized in a particular direction regardless of its composition.